An exhaust gas recirculation device (EGR device) has been widely used which returns a portion of exhaust gas (hereinafter referred to as “EGR gas”) to an intake system to slow combustion and lower a combustion temperature in an internal combustion engine to thereby achieve reduction of NOx.
FIG. 1 shows an example of a supercharged internal combustion engine 1 which has intake and exhaust passages 3 and 5 connected to intake and exhaust manifolds 2 and 4 of the engine 1, respectively. The exhaust passage 5 is provided with a turbine 7 driven by an exhaust gas 6, and the intake passage 3 is provided with a compressor 8 which produces compressed air 9 with the turbine 7. The turbine 7 and the compressor 8 make up a supercharger 10 as a turbocharger so as to supply the air 9 compressed by the compressor 8 of the supercharger 10 to the intake passage 3. In FIG. 1, reference numerals 11, 12, 13 and 14 denote an air cleaner, an intercooler, cylinders of the engine 1 and a fuel injector, respectively.
Disposed between the intake and exhaust passages 3 and 5 is an EGR means 15. The EGR means 15 of FIG. 1 interconnects the intake and exhaust manifolds 2 and 4 through an EGR piping 16 provided with an EGR cooler 17 and an EGR valve 19 opened/closed by an actuator 18.
An accelerator pedal (not shown) of a driver seat is provided with an accelerator sensor (load sensor) 20 which detects an accelerator opening degree as a load of the engine 1. The engine 1 is provided with a rotation sensor 21 which detects a rotation number thereof. Accelerator opening degree and rotation number signals 20a and 21a of the sensors 20 and 21, respectively, are input to a controller 22 which constitutes an engine control computer (ECU: Electronic Control Unit).
On the other hand, the controller 22 outputs an opening degree command signal 18a which gives a command for an opening degree to the actuator 18 of the EGR valve 19 as well as a fuel injection signal 14a which gives commands for injection timing and amount of fuel to the fuel injector 14 for injecting fuel into the cylinders 13.
During an operation of the engine 1, the controller 22 controls the EGR valve 19 on the basis of the accelerator opening degree and rotation number signals 20a and 21a to maintain the opening degree of the EGR valve 19 at a position depending on an operating state of the engine 1, thereby recirculating the EGR gas to the intake passage 3 for reduction of NOx in the exhaust gas.
The controller 22 determines a target EGR ratio on the basis of the rotation number of the engine 1 and the like and controls the EGR valve 19 and the like for conformity with the target EGR ratio. Recently proposed is a device which diagnoses a failure of EGR means due to a defect of an EGR valve (see, for example, Patent Literature 1).